EARTHQUA
P r o p e r design and construction c a n prevent
loss of life and minimize economic loss
Donald E. Hudson
Every year there are about 1,000,000 true earthquakes occurring in the earth. Most of these are so
small that they can be detected only by sensitive
instruments, but about 100,000 of them could be
felt to some degree by human beings located near
the origin. Fortunately, only about 100 are of a size
sufficient to cause severe damage, and most of them
occur far from any man-made structures. As an
average there are each year perhaps a dozen or so
earthquakes that cause significant damage somewhere in the world. These dozen are enough to represent a heavy economic loss and a continuing
to life and limb in many areas of the world.
ing to a recent UNESCO report, earthuakes between 1926 and 1950 resulted in 350,000
uman deaths and an economic loss estimated at
$10 billion.
ough the number of earthquakes is not
o increase significantly, the severity of
the problem is sure to grow, because the world is
rapidly filling up with people and structures. For
example, had the Alaska earthquake of 1964 occurred a few years earlier, there would have been
much less damage, because there would have been
few structures in the area.
Studies based primarily on the work of Caltech
seismologists Charles Richter and the late
Gutenberg and reported in their hook Seismic
the Earth have shown that the distribution of
quakes over the earth is far from uniform. About 80
percent of the world's earthquakes occur in a relatively narrow belt circling the Pacific Ocean. The
most seismic parts of the United States are the Pacific Coast states, which form a segment of this
basic circum-Pacific belt. It cannot be assumed,
however, that destructive earthquakes will not occur in other parts of the conn
quakes occurred in the centr
( 1811-1812) and in Charleston
sharp shocks have been felt in the Boston area
during the past 100 years.
Earthquakes are commonly described by their
magnitude, measured on the Richter scale, but
many of the most destructive in terms
and lives were seisrnologically "modera
A brief survey of recent well-known earthquakes
shows that the Alaska earthquake was perhaps not
quite as large as the 1960 Chile earthquake, and
both were probably exceeded by the Assam [India]
earthquake of 1950. The San Francisco earthquake
of 1906 is definitely down the scale a little, and the
Kern County earthquake of 1952, the most recent
one to cause appreciable damage
was of an intermediate size. Of equ
low end of the scale; Agadir [Moro
[Yugoslavia], which did imrnen
killed thousands of people, were relatively small
earthquakes that happened to occur close to densely populated areas with many very weak structures,
Engineering and Science
Similarly,
bara and Long
Beach ear
eep impression
on the southern California consciousness of the
earthquake hazard, were relatively small. The mimber of small earthquakes far exceeds that of large
earthquakes; there are on the average about 12
shocks of magnitude 6 each year, but only one of
magnitude 8.
A magnitude 8.5 earthquake would cause structural damage over an area of about 100,000 square
miles ( the area of southern California) ; the Long
Beach earthquake (magnitude 6.3) caused damage
over some 300 square miles. In light of the greater
number of smaller earthquakes, however, it can be
concluded that there is almost as much total damage
from smaller earthquakes as from the larger. The
bigger ones nevertheless present more of a problem,
since damage over a very wide area complicates
relief and rescue w
d intensifies the economic
problems of recove
d reconstruction.
Considering past data on the numbers of earthquakes of various sizes occurring in California and
the areas of damage associated with each, one can
arrive at the number of years that should elapse on
the average between destructive ground motions at
any particular point. I t is found for any point in
California that the expected frequency of experiencing ground motion equal to or greater than that
in Long Beach during the 1933 earthquake is about
once per 70 years. Smaller magnitudes of shaking
will be felt more often, but the 70-year figure is a
good one to keep in mind in relation to the expected
life of structures.
One cannot "run away m earthquakes" in California by locating structures far from
there are too many faults distribute
the state. A common opinion now i
the whole of California should be
have approximately the same earthquake risk.
Effectsof earthquakes on structures
tructural damage caused by earthquakes falls
into two broad classes: that caused primarily by a
disruption of the foundation, and that caused by
shaking of the ground. The dangers of foundation
disruption were clearly shown in the Alaska earthquake, where landslides caused great damage. In
one instance a hospital, which withstood the shaking of the earthquake as it had been designed
narrowly escaped destruction when a Ian
came within a few feet of it, Although the hospital
site has obvious scars that are evidence of former
landslides, those warnings were ignored, and a water supply tank was erected right on the edge of a
February 1966
previous slide. This whole situation illustrates one
type of precaution that should certainly be taken
in locating buildings. No amount of skill in structural design could withstand an undermining of the
foundation by such landslides. Other areas in Anchorage were not as fortunate, and a large fraction
of the damage there was the resii t of houses being
engulfed in massive slides.
A second kind of foundation disturbance was
shown in Niigata [Japan] where whole buildings
tilted as though they were rolling in a heavy sea.
One large multi-story apartment house rotated
through an angle of 80 degrees in foundation soil
that was "liquefied" by the earthquake sha
Even though this concrete building finished
its side, little structural damage occurred.
The type of damage that is most susceptible to
analysis occurs when buildings on firm ground are
shaken by an earthquake. Before-and-after photographs of a multi-story office building in Agadir
show what happened during the I960 earthquake.
Although the building disintegrated because of the
shaking of the solid ground, telephone poles remained firmly planted in the ground.
Measurement of destructive grouwl notion
I t might be expected that information about the
shaking of the ground during destructive earthquakes would be obtained from the instruments in
seismological laboratories. However, those instruments are for the most part sensitive devices designed to record distant earthquakes giving rise to
extremely small ground motions. If a strong earthquake should occur near the station, the instruments would read off-scale, or might even, as in
Tokyo in 1923, be thrown off their bases onto the
Even if a structure is well built, a poorly chosen site
can lead to disaster in an earthquake, as almost happened at this hospital in Anchorage. Note the scars
of old landslides to the right of the latest break.
T h i s building in Agadir, Morocco, collapsed during
an earthqt~ake,although the ground remained firm.
Another "strt~ctz~re"
the telephone pole, was unharmed.
floor. Earthquake engineers must thus design and
install their own rugge instruments specifically for
the purpose of recording big earthquakes.
Recording strong grew
A special strong-motion accelerograph has been
designed for recording three components of the
ground acceleration versus time during strong
earthquakes. Because the recording paper rn ust
move fairly rapidly to permit the analysis required by the engineer, it is not feasible to run the
paper continuously as in seismological instruments.
The instrument must be triggered by the earthquake which is to be recorded. This is
starting pendulum, which at the beginning of the
ground motion makes an electric contact to start the
photographic paper and turn on the recording light.
One of the biggest deficiencies of the present en2
gineering studies of earthquakes is the lack of a
sufficient number of such instruments. In order to
give a usable record, the device must be located
within 20 to 30 miles of a large earthquake. The
area to be covered is immense, and thousands of
instruments are needed where only hundreds exist
at present.
Because of this lack of instruments, for only one
recent, destructive earthquake-Niigata-has a record of the strong ground motion occurring in the
region of damage been obtained. Of course this
lack of basic data hampers studies of what happened, since in examining ruins one is often not
sure whether the damage was caused by a heavy
ground motion or by an especially weak structure.
The U.S. Coast and Geodetic Survey maintains a
network of recording instruments, which includes
15 accelerographs in Alaska installed since 1964.
Unfortunately, there were no instruments in Alaska
to measure the destructive ground motions of t
1964 earthquake. Because of the limited area of
coverage of each instrument and the small probability of occurrence of a strong earthquake sufficiently near any particular instrument, such networks must be operated for many years to accumulate useful data. There is a concentration of instruments in San Francisco and Los Angeles where
there are many important structures on various
foundation conditions. In addition, several buildings in San Francisco and Los Angeles have instruments in upper-story positions to record the behavior of the building: during: earthquakes. Such
simultaneous measurements of ground motion and
building response make it possible to consider a
strong earthquake as a full-scale, dynamic test of
the structure, from which significant dynamic properties can be computed.
In spite of the small number of instruments, a
number of excellent records of strong ground motion have been obtained. One, the El Centre 1940
earthquake record, has become in a sense a standard
earthquake for all workers in the field. A technical
paper on the subject from any seismic country (such
) will probably refer to it.
uake had a maximum horizontal acceleration of about one-third the acceleration of gravity. George Housner, Caltech professor of civil engineering and applied mechanics, has
made a special study of the maximum horizontal
acceleration that might be expected close to a fault
urn during an earthquake, an
t it is of the order of one-ha1
celeration of gravity. There are reasons for supposing that the properties of the earth's crust are such
continued on page 24
u
u
A
Engineering and Science
Earthquake -Resistant Design
. . . continued
that more destructive shaking is unlikely or even
impossible. Similarly, the maximum time duration
of strong shaking is of the order of 45 seconds, although the shaking may he felt for a longer period.
Designing for Safety
A typical building, in the absence of earthquakes
or wind, supports its own weight and contents as
vertical loads. The effect of an earthquake is mainly
to apply horizor
loads to the building. As a first
approximation,
se horizontal forces are proportional to the weights of the floors, and also to the
ground acceleration. A secondary effect is a modification of the vertical loads because of vertical
accelerations, but this is not usually as important
a factor as the horizontal force.
To illustrate the potentially destructive nature of
a horizontal force, consider a simple structure
formed by placing a beam on top of two columns
without connections. Such a structure might adequately support a vertical load, but could easily be
toppled by a horizontal load. A simple way to cure
this difficulty would be to connect the members together so that when a horizontal force acts, the
structure perhaps bends, but does not collapse. This
illustrates one of the most important principles
of earthquake-resistant design-that the structure
should be firmly connected together so that it acts
as a unit. This may seem to be such an obvious consideration that it hardly needs to be mentioned.
Nevertheless, failure to remember it is the hasic
cause of much earthquake damage, as was illustrated by a number of instances of complete collapse during the Alaska earthquake.
A second simple type of difficulty may exist when
two different buildings are close together. Being
different, the two buildings are likely to vibrate in
a different way during an earthquake, one zigging
while the other zags. The consequent pounding of
tlie two buildings can do severe local damage, and
this has often been noted in past earthquakes. One
cure, of course, is to provide a sufficient clearance
between the structures to prevent contact.
A rather more complicated difficulty can be illustrated by an L-shaped asymmetrical bui
where one part of the building is much stiffer than
the other. Differences in the way in which the two
sections vibrate may set up a damaging condition at
tlie juncture of the two sections. This does not mean
that asymmetrical buildings should not be built,
but special provisions should lie made to strengthen
them at critical sections.
Much earthquake damage can be traced to a
relatively few basic design errors such as those
just mentioned. The cure for such difficulties is
more widespread dissemination of information
among architects and engineers.
Among the steps taken to ensure safe structures,
perhaps the most important is the establishment of
building codes or regulations containing directions
for earthquake-resistant design and construction. A
properly formulated building code embodying cur:rent knowledge in the field, backed up by lega
enforced inspection and control, can go a long way
toward assuring public safety during earthquakes.
Such building codes are not as common or as comprehensive as is often supposed.
The most widely used building code in the west
is the Uniform Building Code of the Pacific Coast
Building Officials Conference. This code, which
covers all aspects of construction, is is
standard form that can be legally adopt
ous municipalities and government agencies.
present some 650 such agencies have officia
adopted it. However, the existence of the standard
code does not mean that all cities have o
adopted it, or that, if adopted, it is effect!
forced. The universal experience of all countries
has been that an earthquake code itself without a
vigorous and continuous inspection and enforcement policy is of little use. In any given region,
therefore, the question to ask is not whether an
earthquake code exists, but whether or not there is
an active local group backing it.
The first effective earthquake-resistant building
code in California appeared in 1927 in the
edition of the Uniform Building C
the code has been revised every
years to keep up with the increasing knowledge.
The Long Beach earthquake of 1933 was the real
breakthrough in the development of regulation
and control. As a direct consequence of the great
damage to school b
gs in the Long Beach
earthquake, the Leg
e of the State of California, through the Field Act, assigned to the State
Division of Architecture the authority and responsibility, under the police power of the state, to approve or reject plans and specifications a
vise construction of all public school bu
State Division of Architecture has carried this assignment out with great effectiveness, and all California school buildings built since 1933 have been
continued on page 26
Engineering and Science
Earthquake -Resistant Design
. . . coiitmwii
designed and constructed under careful supervision
with respect to earthquake forces.
For many years both San Francisco and Los Angeles operated under building codes different from
each other and from the Uniform Building Code.
In 1960 a special Seismology Committee of the
Structural Engineers Association of California developed a standard earthquake code which now
has almost universal acceptance, so that Los Angeles, San Francisco, and the Uniform Building
have virtually the same earthquake proI t is not to be supposed that an earthquake co
is a comprehensive treatise on earthquake-resistant
design. The complete earthquake section of the
Uniform Building Code runs to only a dozen or so
pages; it is only a guide for a trained and experienced designer. Such building codes must always
presuppose the existence of a high-level professional activity in the area.
Earthquake engineering researc
For many years basic problems of the behavior of
structures subjected to earthquake forces have been
studied at Caltech. One type of problem which has
been solved involves a simple one-stoy structure
consisting of a mass mounted on horizontally
flexible columns. It is supposed that the ground has
the acceleration of a measured past earthquake, and
a calculation of the maximum deflection of the
building with respect to the ground is made. This
maximum deflection is plotted against the ratio of
mass to stiffness, so that for any given structure an
idea of the deformation caused by the earthquake
can be worked out. Whether or not the calcu
deformation is dangerous for the structure m
course be further examined. Analyses have been
made for all of the past strong earthquakes for
which good ground acceleration records are available. By a comparison of the results of such calculations, it has been possible to express certain average properties of past strong earthquakes in a
form that is useful for design purposes.
A second research field in which the Caltech
group has been active is that of the dynamic testing
of full-scale structures. Because of the difficulty and
expense of making tests of large structures, there
are many gaps in the basic knowledge of the dynamic characteristics of such things as multi-story
bridges, and dams. To learn more about
ems, a special system of vibration genera26
tors has been developed at Caltech under the
sponsorship of the California State Division of
Architecture. A test program was carried out in cooperation with the Los Angeles Department of
Water and Power in which a set of four vibration
generators was mounted at the crest of an earthfilled dam. With this system, a sinusoidally varying
horizontal force of 20,000-poun amplitude can be
produced over a range of accurately
frequencies. By measuring the motion
at various frequencies, one can c
namic physical properties of the
constructed condition. Similar tests
in several multi-story buildings and in various special structures, such as large rocket test stands, and
a considerable clarification of the dynamic properties of structures has resulted.
Current design philosophy does not attemp
avoid all damage, but does intend to prevent
kind of complete collapse that would lead to injury
or loss of life. Once this has been accomplished, the
designer hopes to balance repair costs against the
increased initial building costs that would have
been necessary to prevent damage completely. This
is clearly a statistical problem which requires for
its satisfactory solution improvements in knowledge
of the probability of occurrence of earthquakes in
a particular region and of the true dynamic behavior of structures.
If all existing knowledge can be employed and
good construction techniques and high-quality materials can be ensured, there is no reason why structures cannot be made safe in the above sense against
the most violent earthquakes. The principal danger
in the world today comes from the millions of old
structures that were built either before present
knowledge existed, or under economic conditions
that for various reasons did not permit suitable
quality construction. Unfortunately, a large fraction
of the world's population must ive in houses that
would inevitably collapse in e en a small earthquake. This is a problem that is being investigated
by several UNESCO committees, but as yet no
satisfactory solution to this immediate practical
problem seems to be in sight.
Although there are a number of such pressing
practical problems remaining to be solved, it may
be said that the basic knowledge of the effects of
earthq
on structures is at present extensive
and is
ly advancing. It may thus be expected
that, with the proper effort, the world will ultimately be relatively free of serious earthquake hazards.